Pumps designed for pumping fluids such as liquid CO.sub.2 (carbon dioxide) are well known in the art. Such pumps are often used for the purpose of filling cylinders used in the beverage dispensing, fire extinguishing and welding industries. These pumps are typically self-contained units connected by intake and discharge tubing to a supply tank and to a cylinder which is to be filled and pressurized by the pump. The pumps typically include a sealed crankcase housing containing a shaft-driven bearing which reciprocates a piston in a cylinder, with a pump head mounted on one end of the housing to draw CO.sub.2 from the supply tank through an intake valve and to discharge it on the compression stroke through a discharge valve. Especially in food grade, medical, laboratory and welding applications, the pump housing must be carefully sealed to prevent lubricating oil from contaminating the cylinder being filled. The crankcase housings for the pumps have typically been costly to manufacture and difficult to seal, comprising large, relatively complicated cast or machined parts.
In food grade applications it is actually preferred to use oil-free pumps because of the risk of oil contamination, and for this purpose manufacturers typically offer both oil-using and oil-free pumps to their customers, depending on the application. Manufacturing two different styles of pump for oil-lubricating and oil-free use increases cost both to the manufacturer and to the customer.